An optical waveguide is a light pipe capable of guiding a light signal along a path that can include curves, loops, etc. without a significant loss of optical energy from the signal. Typically, a waveguide includes a central core of substantially transparent material that is surrounded by cladding material whose refractive index that is lower than that of the core material. This refractive index difference contains the bulk of the optical energy of the signal within the waveguide core. The most common optical waveguides include glass-based optical fibers (typically used for simple transmission of light from one place to another) and surface waveguides formed on rigid substrates, such as glass or silicon. Surface waveguides offer the promise of greater functionality than simple optical fibers by virtue of their ability to readily include more complex structures, such as ring resonators, 1×N couplers and splitters (where N can be 2, 3, or more), and the like, which are difficult to realize in optical fibers. Often, multiple surface waveguides are formed on a single substrate to collectively define a planar lightwave circuit (PLC).
The “mode” of the light signal propagates primarily within the core, although a portion (its “evanescent field”) extends into the cladding. The shape of the mode and the size of the evanescent field depend strongly on the design of the waveguide. Factors such as waveguide design (i.e., cross-sectional shape), index contrast (i.e., the refractive index difference between the core and cladding), core size, and cladding thickness all impact how strongly the light is confined in the core, as well as the shape and size of the optical mode (i.e., mode profile and mode-field size).
In waveguides having only a small difference between the refractive indices of the core and cladding material (referred to as “low-contrast waveguides”), light is loosely confined in the core and the evanescent field is relatively large. The optical propagation loss of such waveguides can be very low; therefore, low-contrast waveguides are preferred in applications where low propagation loss is critical, such as for transmission in optical telecom or datacom systems. In such systems, the vast majority of waveguides used are “communications-grade” optical fibers, which are typically characterized by very low contrast and very low optical propagation loss. Such optical fibers typically provide little optical functionality other than simply conveying light signals from one place to another. Surface waveguides, however, offer greater functionality and can be fabricated in a range of index contrasts from low-contrast surface waveguides through high-contrast surface waveguides.
Low-contrast surface waveguides typically exhibit optical propagation loss that is somewhat higher than that of a typical communications-grade optical fibers, but can also enable low-loss waveguide crossings, optical power splitting, and optical power coupling, which are difficult to achieve using optical fibers. Unfortunately, because they only loosely confine light signals, low-contrast surface waveguides are susceptible to severe losses at waveguide bends, as well as disruption from optical signals propagating in nearby low-contrast surface waveguides. Low-contrast surface waveguides, therefore, typically require large bending radii and are not well suited for use in high-density PLCs. As a result, low-contrast surface waveguide systems require a large chip area, which increases their cost. A low-contrast surface waveguide can be designed with a propagation mode that substantially matches the mode profile and mode-field size of an optical fiber, however, which reduces the optical loss that arises when a light signal is transferred between the surface waveguide and the optical fiber. They are attractive, therefore, for combined systems where a low-contrast surface waveguide is optically coupled with an optical fiber to add functionality to a low-loss optical system.
High-contrast surface waveguides (i.e., surface waveguides having a large difference between the refractive indices of the core and cladding material) tightly confine a light signal in the core such that its evanescent field is relatively small. This enables high-contrast surface waveguides to have extremely small bending radii and be located quite close to other high-contrast waveguides without incurring significant signal degradation. As a result, high-contrast waveguides enable complex circuit functionality in a small chip area and are well suited to large-scale integration PLCs having densely packed surface waveguides. Unfortunately, high-contrast waveguides typically have relatively higher optical propagation loss. Their use, therefore, has historically been limited to applications where functionality is more important than low loss, such as sensors, power splitters, and the like. In addition, the mode profile of a high-contrast surface waveguide is not well matched to that of an optical fiber; therefore, the optical loss that arises when a light signal is transferred between a high-contrast surface waveguide and an optical fiber is typically quite large. As a result, they are not well suited for combined systems where a high-contrast surface waveguide is optically coupled with an optical fiber.
In some cases, it is desirable to have both high-contrast surface waveguides and low-contrast surface waveguides in the same PLC. One way to enable this is through the use of a spotsize converter, sometimes referred to as a mode-field converter. In addition, a spotsize converter can enable the use of a high-contrast waveguide-based PLC with a conventional optical fiber by changing the mode profile of the high-contrast surface waveguide to more closely match that of the optical fiber, thereby reducing fiber-to-chip coupling loss.
Attempts to form a PLC-based spotsize converter in the prior art have typically relied on waveguides regions comprising a one-dimensional taper in the lateral dimension, wherein the lateral taper is formed using conventional photolithography and etching. Examples of such devices are described in “Optical spotsize converter using narrow laterally tapered waveguide for Planar Lightwave Circuits,” J. Lightwave Tech., Vol. 22, pp. 833-839 (2004). While some improvement in coupling performance is achieved with this approach, the performance and flexibility of these devices is limited because the mode-field can only be controlled in one dimension.
Silicon-core waveguides whose cores are tapered in one dimension have also been investigated in the prior art, such as is described in “Spotsize converters for rib-type silicon photonic wire waveguides,” Proceedings of the 5th International Conference on Group IV Photonics, Sorrento, Italy, September 17-19, pp. 200-202 (2008) and “Low loss shallow-ridge silicon waveguides,” Optics Express, Vol. 18, No. 14, pp. 14474-14479 (2010). Unfortunately, while the promise of compatibility with conventional integrated circuits is attractive, the operating wavelengths and propagation losses for silicon-core waveguides limit their use in many applications.
In similar fashion, optical coupling between an optical fiber and a photonic crystal waveguide via a laterally tapered silicon-wire waveguide region was demonstrated in “Spotsize converter of Photonic Crystal Waveguide,” NTT Technical Review, Vol. 2, pp. 36-47 (2004).
Of more promise, however, are mode-field conversion regions formed in waveguides that are tapered in two dimensions, such as described in “Low-Loss Compact Arrayed Waveguide Grating with Spot-size Converter Fabricated by a Shadow-Mask Etching Technique,” Electronics and Telecommunications Research Institute (ETRI) Journal, Vol. 27, No. 1, pp. 89-94 (2005). While the structure of these spotsize converters shows great promise for low fiber-to-chip coupling losses, shadow-mask etching is extremely difficult to control. As a result, spotsize converters fabricated in this manner are expensive to produce in volume and are likely to suffer from variations in performance as well, making them difficult, at best, to commercialize.
An improved method forming low-cost, commercially viable spotsize converters that are operable over a wide range of wavelengths would, therefore, be highly desirable.